Property Search Eye dropper
There is currently no native way to do it. prop_search is a generally bad idea for linking objects since it stores only the name of the object, once you change it in the outliner the reference will get lost. I am using a custom made property for that purpose which stores a pointer to the object and it has a custom made eye dropper as well. https://github.com/WowDevTools/Blender-WMO-import-export-scripts
You can find it in this addon in idproperty folder. Let me know if you have any questions on how to use it. Besides, I heard they are adding proper object pointers into 2.79, so once it is out, you may consider replacing those custom properties with the official ones.